The present invention relates to new and useful polymers, and more specifically, to improved nonleachable, optically transparent, homogeneous radiopaque heavy metal-containing polymer complexes, compositions of matter and articles prepared therefrom.
Translucent polymeric materials, and particularly, acrylic type resins have been widely used for years in both medical and dental applications. In dentistry, for instance, plastics are used in a broad range of materials and appliances, including removable dentures, temporary crown and bridge materials, restorative materials, impression materials, and the like. They also find many applications in medicine, such as surgical and body implants and other prosthetic devices, e.g. heart valves, blood vessels, etc. Translucent plastics are also widely used in medical appliances, such as catheters.
The desirability of imparting radiopacity to plastics used in dentistry and medicine has been recognized. In dentistry, for example, it has been difficult to detect secondary caries or underlying decalcified dentin resulting from the placement of unreinforced direct restorative resins because these materials are relatively radiolucent, and are not opaque to x-rays. Surveys have also shown that dental instruments, materials and nonfixed appliances have fractured and become embedded in soft tissues, ingested or inhaled inadvertently by patients. Although incidences of ingestion or inhalation of dental plastics are relatively rare compared with other foreign objects, the occurrence may result in a severe medical emergency or even death. The potential severity of such an incident makes it imperative to diagnose and remove such foreign bodies rapidly.
In medicine, hip joint replacements require cementing. Therefore, it would be desirable to monitor the positioning of bone cement without surgical procedure. Similarly, the ability to monitor by x-ray replacement heart valves, arteries, including the path of catheters traversing blood vessels and organ systems would be desirable. Hence, there is a need for polymeric materials with increased radiation absorption potentials which also possess the requisite nonleachable properties for safe and acceptable use in dentistry and medicine.
Heavy metal salts, like those of bismuth, barium, etc., have been used as contrast medium in diagnostic radiography. They have properties which would suggest their suitability for increasing the radiation absorption potential of medical and dental resins. As a result, substantial effort has been made to incorporate barium sulfate and other radiopaque salts, like bismuth bromide, bismuth chloride, bismuth subnitrate, etc., into polymers to render them opaque to x-rays. However, earlier radiopaque polymers containing heavy metal salts have not been totally satisfactory. Heavy metal-containing radiopaque materials fall into two principal groups: radiopaque glass containing embedded heavy metals, and polymers mixed with heavy metal salts. In the case of heavy metals embedded into radiopaque glasses, the metal is not molecularly bound to the polymer matrix, and therefore, has a tendency to weaken the composite. Moreover, because glass filler based resins lack homogeneity a further weakening of regions in the matrix results. Those regions of a composite having little or no glass are radiolucent. In addition, a light scattering effect is produced by radiopaque glasses which alters optical properties and renders them optically opaque.
Heretofore, polymers with added inorganic heavy metal salts were essentially physical mixtures, present as fine powders locked in a matrix. Their preparation resulted in an uneven distribution of the salt, and had an adverse affect on the mechanical properties of the plastic material. The salt gradually leached out of the matrix causing discoloration of the polymer and release of heavy metal toxins. The salt and polymer remained as separate distinct phases producing an opaque, cloudy material which scattered light. Mixing does not impart homogeneity between the salt and polymer.
Examples of bismuth salt-containing polymers are disclosed by E. C. Combe in the Dental Practitioner, 51-54 (1971); Journal of Dental Research Supplement, Vol. 50, 1192 (1971) and Journal of Dentistry, Vol. 1, 93-97 (1972). They are also described by Elzay et al in the Journal of Prosthetic Dentistry, Vol. 25, 251-257 (March 1971). Combe reported in the Dental Practitioner supra tests with a radiopaque self-curing crown and bridge resin then available from Coe Laboratories under the Raypaque trademark. This composition apparently comprised a heterogeneous mixture of bismuth tribromide and a polymer. Combe concluded that although Raypaque resin had greater radiopacity than other materials, the bismuth tribromide polymer mixture had lower impact strength than conventional radiolucent acrylics. Combe also reported in the Journal of Dental Research supra in 1971, that studies of 12 percent bismuth trichloride added to acrylic dough provided radiopaque polymers with transverse strength which closely approached that of the unmodified acrylic.
The 1972 Journal of Dentistry paper supra reported studies in which 8.1; 15.3; 21.7 and 26.5 percent (w/w) bismuth trichloride was dissolved in methyl methacrylate monomer before preparation of the dough. However, of the amounts listed only 8.1 percent bismuth trichloride fully dissolved in the monomer. That is, radiopaque polymers comprising bismuth trichloride in amounts of 15 percent and more did not fully dissolve in the methyl methacrylate monomer, and consequently, such nonhomogeneous polymers had significant amounts of uncomplexed, leachable heavy metal salts. In addition, the composition comprising only 8.1 percent bismuth trichloride had an inadequate amount of heavy metal to impart sufficient radiopacity properties to the polymer so it was at least equivalent to that of aluminum. The composition originally containing 8.1 percent bismuth trichloride was diluted even further when it was swelled in additional methyl methacrylate monomer in making the dental dough, resulting in a further reduction in radiopacity properties. Combe ultimately concluded that in terms of affects on mechanical properties there was little to choose. The bismuth halide polymers were also rejected because of high water absorption of the acrylics containing these additives.
Elzay et al reported on a series of heavy metal-containing heterogeneous radiopaque polymeric materials, including two prepared with bismuth subnitrate for dental prosthetic devices. The bismuth subnitrate and barium sulfate containing polymers were found unacceptable by Elzay et al as radiopaque additives for acrylic resins because of a staining effect in 26 percent of the dentures tested.
Additional radiopaque contrast media have been formed by the use of (a) other heavy metal salts, such as silver and lead; (b) heavy metals embedded in silica filler and then added to the resin composition, and (c) the use of highly halogenated polymers. Halogenated polymers or halogenated organic additives, e.g. brominated polymers have good physical properties, but the halogen functional groups tend to hydrolyze or decompose and form leachable compounds. This is especially pronounced in acidic aqueous fluids as commonly found in humans. In addition to the potential toxic effects, leaching of bromide and other halogens causes discoloration of the resin which gradually converts the polymer to a radiolucent material. The halogenated organic additives can also act to decompose the polymer. Accordingly, there is a need for improved radiopaque materials where a radiopacifying salt is molecularly bound to the polymer to form clear, homogeneous, nonleachable materials with mechanical properties which are substantially equivalent to those polymers which are free of such salts.